1. Field of the Invention
This invention relates to television systems. In a primary application the invention relates to increasing the aspect ratio of compatible high-definition television systems.
2. Description of the Prior Art
With the advent of improved display and camera tubes, efforts have been made to provide high definition television (HDTV) systems for home consumption. It is clearly desirable to have this improved system compatible with existing television receivers and channel allocations. A major problem in this compatibility requirement is that existing systems and receivers are based on a 4/3 aspect ratio, while the newer high definition systems have higher aspect ratios of approximately 5/3. A number of approaches have been suggested to deal with this problem. These are described in the December 1987 issue of the IEEE Transactions on Broadcasting Vol. BC-33, No. 4. In an article by M.A. Isnardi et al. of the David Sarnoff Research Center on pages 116-123 of that issue a compatible system is described. It provides the additional information to add added material on either side of the existing image to increase the aspect ratio. This is done by first time-compressing the low frequency information in these "side panels" so that it occupies a negligible width, and can thus be added to the existing image without being disturbing. The 1.0 .mu.sec. on each end of a horizontal line are normally hidden by the overscan of most receivers. The high frequency information of these "side panels" is time expanded to fill the entire line, thus reducing its bandwidth. This additional information is sent on an additional subcarrier at 3.1 MHz. This subcarrier is rendered invisible on conventional receivers by having it interlaced (odd multiple of one-half the line frequency) and phase-reversed on alternate fields so as to average to zero in the color channel. This approach, although effective, uses up a significant portion of the available spectra for adding information. Therefore, the system described only modestly increases the resolution over existing systems.
In the same issue of the IEEE Transactions another method is described for increasing the aspect ratio in an article by W. E. Glenn and K. G. Glenn on pages 107-115 entitled "High Definition Television Compatible Transmission System". Here the authors make the astute observation that the desired aspect ratio increase can be realized by making the horizontal extent 10% greater, and the vertical extent 10% less. However, in making use of this observation, the authors suggest running video from the end of the burst interval, to the beginning of the next sync pulse, adding about 4 .mu.sec. To reduce the vertical size, 43 of the active vertical lines are inactivated. Firstly, it is indeed highly questionable whether receivers will operate properly with this greatly reduced blanking interval. Many will exhibit foldover, sync and color problems. Also, it is inefficient to have 43 lines without any information on them when trying to get the greatest resolution in a limited bandwidth.